From Brotato Wiki
Bag
Item
Tier 1
Stats
+15 materials when you pick up a crate
-1% Speed
-1% Speed
Price
15
Limit
3
Item
Bag is an in-game item that can be bought from a Shop or can be found in a Crate. Most Items modify the Stats of the player, and some of them provide additional effects.
Bag
Bag provides 15 Materials for each Loot Crate you pick up. You can have up to 3 bags, providing you a total of +45 materials per loot crate. Since multiple crates can be found per wave, bag can end up becoming a significant boost to your economy.
Unlike the Materials gained by Harvesting, Bag doesn't give Experience. It only gives you the Currency Part of Materials.
You can increase the amount of Loot Crates you find by buying the 
Tree items, as trees have a decent chance to drop crates, and by increasing your Luck stat.